Method for producing stainless steels, in particular high-grade steels containing chromium and chromium-nickel

ABSTRACT

The invention relates to a method for producing stainless steels, in particular steels containing chromium and chromium-nickel. The method is carried out in a melting device containing a metallurgical vessel, or in a melting device ( 1 ) containing at least two vessels ( 2, 3 ) for supplying a steel-casting installation, an electric arc furnace process ( 1 ) and an air-refining process taking place alternately in the two vessels ( 2, 3 ). To improve the efficiency of a method of this type, the aim of the invention is to carry out a reversible treatment of unreduced converter slag in the electric-arc furnace mode. To achieve this, in the first treatment stage, the slag ( 19 ) with a high chromium content is melted together with the added charge, the slag is then reduced during the melting process with the silicon and carbon under favourable thermodynamic conditions of the arc, once the slag has reached a minimum temperature of 1,490° C. and the slag is subsequently removed. The air-refining process is then carried out, during which the carbon content is reduced to a value of less than 0.9%. The metal slag ( 18 ) is tapped at a tapping temperature of between 1,620 and 1,720° C., the unreduced slag ( 19 ) with a high chromium content from the air-refining process remaining in the treatment vessel.

[0001] The invention concerns a process in accordance with the introductory clause of claim 1 or claim 2.

[0002] Multistage processes in melting equipment that comprises at least two vessels are well known for the production of high-grade steels that contain chromium or chromium-nickel. Depending on the particular process technology, decarbonization is carried out down to carbon contents of less than 0.3%. There is always a high energy requirement, and temperature losses are unavoidable.

[0003] A process of this type is known from DE 196 21 143. The process described there is carried out in melting equipment that comprises at least two vessels. The two vessels are operated in parallel in such a way that alternately either electrodes can be used for melting the charge or blowing lances can be used for top blowing and/or injection of oxygen and oxygen mixtures. The vessels are thus used first as melting equipment and then as decarbonization equipment. After the blowing, the slag is reduced with reducing agents, such as ferrosilicon, aluminum, or secondary aluminum, with the addition of fluxes, such as lime and fluorite, for the recovery of oxidized chromium, and the slag is then tapped. The goal of the invention is to make a process of this type more economical.

[0004] This goal is achieved by the process steps specified in the characterizing clause of claim 1 or claim 2. Effective modifications of the process are contained in the secondary claims.

[0005] The essence of the invention is the reversible treatment of unreduced converter slag in electric-arc furnace operation. In contrast to the well-known process, in which the reduction of the high-chromium slag and thus the recovery of the metallic chromium is carried out in a process step that follows, and is separate from, the melting and oxygen blowing, in this case the reduction is carried out simultaneously with a renewed melting operation of a new charge with retention of the slag of the preceding air-refining process in the vessel. In this way, a process step, namely, the subsequent reduction of the slag, is saved, and the chromium-containing slag is not removed from the system. All together, this makes the process simpler and more economical.

[0006] In detail, the following steps are carried out:

[0007] (a) heating of high-chromium slag in the first treatment step together with the melting down of the added charge, specifically, by electric energy from the electric arc;

[0008] (b) reduction of the high-chromium slag during the melting process with the silicon and carbon under favorable thermodynamic conditions of the arc, after the melt has reached a minimum temperature of 1,490° C., with subsequent removal of the slag;

[0009] (c) treatment of the melt in the same vessel with an air-refining process, in which the blowing of oxygen or oxygen mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination, results in the melt being decarbonized to a carbon value of <0.9%, and preferably <0.4%, and heated to a tapping temperature of 1,620 to 1,720°;

[0010] (d) thorough mixing of the melt with an inert gas, which is introduced through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination;

[0011] (e) injection/top blowing of alloying agents, fluxes, reducing agents, metal oxide/metal-containing dusts or mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination; and

[0012] (f) subsequent tapping of the melt, during which the unreduced high-chromium slag of the air-refining process remains in the treatment vessel and is reduced in the next cycle of the electric-arc melting process in accordance with step (a).

[0013] The proposed process can basically be carried out in a single metallurgical vessel. To accelerate the tapping times, it is proposed, in accordance with claim 2, that the process be carried out with two alternately operated metallurgical vessels. In this case, while the decarbonizing blowing of the charge is being carried out in the first treatment vessel, the melting process of a second charge, including the reduction process of the slag, is being carried out at the same time in the second treatment vessel.

[0014] The melting operation may also be carried out by means other than electric arcs, although it is important to make sure that the favorable thermodynamic conditions for the reduction of the slag are maintained.

[0015] The blowing of oxygen or oxygen mixtures is preferably carried out in the form of top blowing or side blowing. For the purpose of more thorough mixing and homogenization of the melt, inert gases can be blown in at the same time that the oxygen blowing process is being performed.

[0016] At an oxygen blowing time of 20 to 40 minutes, the melt is decarbonized to a final carbon content of <0.9%, and preferably <0.4%.

[0017] During the oxygen blowing, coolants are added, for example, in the form of Ni, FeNi, ferrochromium, scrap, and other iron-containing metallic raw materials, such as pigs, DRI, or alloying agents, to reach the target temperature.

[0018] In accordance with a preferred process step, the air-refining process is ended at a carbon content of less than or equal to 0.9%, preferably less than or equal to 0.4%, and a temperature of more than 1,680° C., and the molten metal is tapped into the ladle. In accordance with the invention, the slag remains in the vessel, in which it is then reduced during the renewed melting operation. Separately from this, in the further course of treatment, the molten metal is brought to the desired final carbon content of <0.1% by a secondary metallurgical treatment, preferably vacuum degassing. This also has the advantage of protecting the refractory material of the vessel, which is extremely stressed during an air-refining process down to low carbon contents.

[0019] In accordance with the invention, the high-chromium slag is reduced with the silicon or carbon from silicon-containing or carbon-containing alloy carriers in the charge. In accordance with an especially preferred variation of the process, it is proposed that additional carbon and possibly silicon be added. The chromium oxide contained in the high-chromium slag is directly reduced to metallic chromium by the carbon and the silicon.

[0020] During the melting down of the charge, oxygen or oxygen mixtures are added through top lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination, to improve silicon and carbon oxidation.

[0021] Additional details and advantages of the invention are apparent from the following description, in which the embodiment of melting equipment for the process of the invention, which is shown in the drawing, in this case with two metallurgical vessels, is explained in greater detail. In this regard, besides the combinations of features enumerated above, features alone or in other combinations are also included in the invention. The single drawing shows a side view of melting equipment with two treatment vessels.

[0022] The melting equipment 1 consists of two treatment vessels 2, 3, in which an electric-arc furnace process (I) and an air-refining process (II) are alternately operated. In the left treatment vessel 2, the operating state of melting by electric arcs is shown, and in the right treatment vessel 3, the operating state of decarbonization or oxygen blowing for reducing the carbon content of the melt is shown.

[0023] For the oxygen blowing, a lance bracket 5 supports a lance 4, which runs coaxially to the main axis of the vessel through an exhaust gas manifold 6 and the roof center opening 7 of an inclined roof 8 of the right treatment vessel 3 and into the interior of the upper part 9 of the vessel. The mouth 10 of the exhaust gas manifold 6 rests on the roof center opening 7 of the roof 8. The upper part 9 and the lower part 11 together form the furnace vessel 3. The exhaust gas manifold 6 can be swung horizontally by a slewing mechanism 12 to the adjacent treatment vessel 2. The lower part 11 has a taphole 13 for the molten metal, here for bottom tapping, while the chromium-containing slag remains in the vessel.

[0024] In the bottom or in the wall of the vessel, there are, individually or in combination, bottom tuyeres 22, porous plugs, side below-bath tuyeres, side tuyeres 20, and/or side lances 21, through which the oxygen, inert gas, and/or gas mixtures are blown.

[0025] The treatment vessel 2 shown on the left has an electrode arm 14 that can be swung horizontally, on which, in the present case, three electrodes 15 a, b, c are mounted, which pass through the roof center 16 of the left treatment vessel 2, which closes the roof center opening 17.

[0026] After the molten metal 18 in one treatment vessel has been tapped through the taphole 13, a new melting operation is begun. The tapped melt is conveyed to a steel casting plant or a secondary metallurgical treatment plant (not shown). Material is charged onto the untapped slag 19 remaining in the vessel; the charge contains especially carbon-containing and silicon-containing raw materials, and the entire contents are then melted down. During the melting process, the high-chromium slag is reduced, after the melt has reached a minimum temperature of 1,490° C. After a minimum temperature of preferably 1,550° C. has been reached, the slag is removed, and the melt is subjected to an air-refining process, which causes the melt to be decarbonized down to a carbon content of <0.9%, and preferably <0.4%, and heated to a tapping temperature of 1,620 to 1,720° C. To this end, the electrode arm 14 is swung out, and the oxygen lance 4 is swung in. Then only the molten metal is tapped. The lance 4 is moved out, and the process starts over again. In the adjacent treatment vessel, this operation is carried out by shifting over time. 

1. Process for producing stainless steels, especially steels containing chromium and chromium-nickel, in melting equipment that includes a metallurgical vessel for supplying a steel casting plant, in which an electric-arc furnace process and an air-refining process are operated in the vessel, and in which in a first of these treatment steps, in which the electric-arc melting process is carried out, a charge that consists basically of solid and/or molten pig iron and raw materials, especially scrap and in part alloy carriers that contain carbon and silicon, is melted down, and the melt is then decarbonized, characterized by reversive treatment of unreduced slag after the air-refining process in the electric-arc furnace operation with the following steps: (a) heating of high-chromium slag in the first treatment step together with the melting down of the added charge; (b) reduction of the high-chromium slag during the melting process with the silicon and carbon under favorable thermodynamic conditions of the arc, after the melt has reached a minimum temperature of 1,490° C., with subsequent removal of the slag; (c) treatment of the melt in the same vessel with an air-refining process, in which the blowing of oxygen or oxygen mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination, results in the melt being decarbonized to a carbon value of <0.9% and heated to a tapping temperature of 1,620 to 1,720° C.; (d) thorough mixing of the melt with an inert gas, which is introduced through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination; (e) injection/top blowing of alloying agents, fluxes, reducing agents, metal oxide/metal-containing dusts, or mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination; and (f) subsequent tapping of the melt, during which the unreduced high-chromium slag of the air-refining process remains in the treatment vessel and is reduced in the next cycle of the electric-arc melting process in accordance with step (a).
 2. Process for producing stainless steels, especially steels containing chromium and chromium-nickel, in melting equipment that includes at least two vessels (2, 3) for supplying a steel casting plant, in which an electric-arc furnace process (I) and an air-refining process (II) are alternately operated in both vessels, and in which in a first of these treatment steps, in which the electric-arc melting process (I) is carried out, a charge that consists basically of solid and/or molten pig iron and raw materials, especially scrap, and in part alloy carriers that contain carbon and silicon, is melted down, and the melt is decarbonized, and in which, at the same time, besides the decarbonizing blowing of the charge in the first treatment vessel (2), the melting process of a second charge is being carried out in the second treatment vessel (3), characterized by reversive treatment of unreduced slag (19) after the air-refining process in the electric-arc furnace operation with the following steps: (a) heating of high-chromium slag (19) in the first treatment step together with the melting down of the added charge; (b) reduction of the high-chromium slag during the melting process with the silicon and carbon under favorable thermodynamic conditions of the arc, after the melt has reached a minimum temperature of 1,490° C., with subsequent removal of the slag; (c) treatment of the melt in the same vessel with an air-refining process, in which the blowing of oxygen or oxygen mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination, results in the melt being decarbonized to a carbon value of <0.9% and heated to a tapping temperature of 1,620 to 1,720° C.; (d) thorough mixing of the melt with an inert gas, which is introduced through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination; (e) injection/top blowing of alloying agents, fluxes, reducing agents, metal oxide/metal-containing dusts or mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination; (f) subsequent tapping of the melt (18), during which the unreduced high-chromium slag (19) of the air-refining process remains in the treatment vessel and is reduced in the next cycle of the electric-arc melting process in accordance with step (a); and (g) such that, at the same time, besides the decarbonizing blowing of the charge in the first treatment vessel, the melting process of a second charge, including the reduction process of the slag, is being carried out in the second treatment vessel.
 3. Process in accordance with claim 1 or claim 2, characterized by the fact that the blowing of oxygen or oxygen mixtures through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination, is carried out in the form of top blowing and/or injection.
 4. Process in accordance with any of claims 1, 2, or 3, characterized by the fact that for the purpose of thorough mixing and homogenization of the melt, the inert gases are blown in at the same time as the oxygen blowing process through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination.
 5. Process in accordance with any of claims 1 to 4, characterized by the fact that, during oxygen blowing for a time of 20 to 40 minutes, the melt is decarbonized to a final carbon content of <0.9%.
 6. Process in accordance with any of claims 1 to 5, characterized by the fact that coolants are added during the oxygen blowing.
 7. Process in accordance with any of claims 1 to 6, characterized by the fact that the air-refining process is terminated at a carbon content of <0.9% and a temperature of more than 1,680° C.; that the molten metal (18) is emptied into a ladle, while the slag (19) remains in the vessel; and that, in the further course of treatment, the molten metal is brought to the desired final carbon content of <0.1% by a secondary metallurgical treatment, preferably vacuum degassing.
 8. Process in accordance with any of claims 1 to 7, characterized by the fact that additional carbon and/or silicon and/or other reducing agents are added.
 9. Process in accordance with any of claims 1 to 8, characterized by the fact that the chromium oxide and other metal oxides contained in the high-chromium slag (19) are directly reduced to metallic chromium and other metals by the carbon and the silicon.
 10. Process in accordance with any of claims 1 to 9, characterized by the fact that, during the melting down of the charge, oxygen for the oxidation of silicon and carbon is added through top lances, side lances, side below-bath tuyeres, side tuyeres, bottom tuyeres, or porous plugs, individually or in combination. 